It has been proposed to improve the capacity of communication by use of spatial diversity or spatial multiplexing. By using spatial multiplexing, the data rate can be increased by transmitting independent information streams from different antennas but using the same channel as defined by frequency, time slot and/or spreading code.
These systems may be referred to as multiple input multiple output (MIMO) systems where there is more than one transmitting element and more than one receiving element with the spatial multiplexing being the various wireless transmission paths from each of the transmitting elements to each of the receiving elements. These systems require complex controllers to control both the transmitting and receiving elements in apparatus such as base stations and mobile stations using MIMO methods.
Multi-stream single user MIMO transmission has been proposed and forms part of Wideband Code-Division Multiple Access (WCDMA), Third Generation Partnership Project Long Term Evolution (3GPP LTE) and Worldwide Interoperability for Microwave Access (WiMax) system standards. In single user multiple input multiple output (SU-MIMO), a MIMO receiver with multiple antennas and receiving circuitry receives the multiple streams, separates the multiple streams and determines the transmission symbols sent over each stream of the spatially multiplexed data streams.
The application of MIMO methods to wireless communication and in particular to Wideband Code Division Multiple Access (WCDMA) high speed downlink packet access (HSDPA) systems such as the double-transmitter antenna array (D-TxAA) adopted by the third generation partnership project (3GPP) enables a significant increase in data throughput and link range without additional bandwidth or transmit power requirements. These systems thus operate with a higher spectral efficiency (in other words, operate with a greater bits per second per Hertz of bandwidth) than conventional HSDPA implementations and also have a higher link reliability or diversity (in other words reduced susceptibility to fading).
However the application of MIMO systems to WCDMA HSDPA systems is problematic with regards to calculating or estimating the Signal to Interference and Noise Ratio (SINR). In particular it is not practical to use the same methods used in conventional HSDPA approaches to estimate the SINR for D-TxAA HSDPA modes of operation. In conventional WCDMA implementations the SINR is calculated using pilot symbols with a known pattern and signal strength and measuring the difference between the received and expected symbols.
In a MIMO implementation data is typically split into at least two streams and the data symbols are pre-coded with the help of pre-coding weights whilst the pilot symbols are transmitted on a separate channel, the Common Pilot Channel (CPICH), without pre-coding. As the CPICH pilot symbols are not pre-coded, it is not possible to use the conventional SINR estimation methods described above.